1. Field of the Invention
The present invention relates to an interlayer for safety glass and, more particularly, to a self-adhering, composite interlayer including a polyvinyl chloride to containing layer.
2. Summary of Related Art
Safety glass is a well known term for a glass sandwich composed of an interlayer bonding together two glass plates or sheets so that breaking the glass results in minimum dispersion of broken glass fragments. The interlayer must possess a number of properties, including the following: high impact energy adsorption to minimize concussive injury; shear and tear strength sufficient to prevent rupture of the interlayer by the broken glass; sufficient adhesion to the glass to inhibit laceration on contact with, and prevent dispersion of, broken glass; acceptable thermal heat stability and weatherability; and good optical quality. The interlayer must possess these properties over the wide range of temperatures in which these laminated glazings are used.
It has been widely known to use a film of plasticized polyvinyl butyral as the interlayer material in safety glass for automobiles, air planes and building materials because of its high adhesiveness, transparency and good mechanical properties over a broad range of temperatures. However, the use of plasticized polyvinyl butyral films makes the production of laminated safety glass relatively expensive.
The surface of plasticized polyvinyl butyral film is very tacky, and presents a problem of blocking at the time of windup after film formation. Thus, plasticized polyvinyl butyral film must be provided with some parting means if it is to be stored or transported in the form of stacks of die-cut blanks or in the form of rolls. Furthermore, the production of plasticized polyvinyl butyral films requires specialized equipment and, due to their sensitivity to moisture, plasticized polyvinyl butyral films must generally be handled under controlled atmosphere conditions during manufacture, storage and immediately prior to their incorporation into the laminated safety glass. This all adds to the expense of utilizing plasticized polyvinyl butyral films in laminated safety glass.
Alternative interlayer materials have been proposed. For instance, U.S. Pat. No. 4,277,538 to Beckmann et al discloses a laminated safety glass employing a sheet of plasticized polyvinyl chloride (PVC) as the interlayer. The use of PVC would be advantageous in that it may be produced on conventional equipment and would be much less expensive to manufacture and process into a laminated safety glass when compared with polyvinyl butyral. However, by itself, a PVC film will not adhere to glass. To increase the adhesion of the PVC to glass, Beckmann et al suggest the use of an organofunctional silane, either as a primer or uniformly dispersed within the PVC film.
However, neither use of a silane adhesion promoter is without drawbacks. Dispersing an organofunctional silane within the PVC film in quantities sufficient to provide adequate adhesion to glass has been found to result in a laminate having a haze which is unacceptably high for many applications. In addition, dispersing the silane in the interlayer may have a negative effect on the processability of the interlayer material. Secondly, the application of an organofunctional silane adhesion promoter as a primer necessitates an additional lamination manufacturing step, increasing the manufacturing costs of the resulting laminated glazing unit.
U.S. Pat. No. 4,600,627 discloses a laminated glazing unit including a pair of glass sheets and a composite interlayer. The interlayer is formed of an interleaving sheet formed from polyester or PVC sandwiched between two intermediate layers of a crosslinking polymer, such as ethylene/vinyl acetate (EVA). The intermediate layers, which provide an adhesive bond between the interleaving sheet and the two glass sheets, have a thickness of about 0.2 mm. It has been determined, however, that that intermediate layers of EVA will not provide sufficient adhesion to the glass, and that the use of silane adhesion promoters is therefore required. In addition, at least where the interleaving sheet is formed of PVC, the resulting laminating glazing exhibits poor optical quality.
Accordingly, it would be advantageous to provide an improved PVC containing interlayer which was relatively inexpensive and self-adhering to a glass sheet. It would further be advantageous to provide such a PVC containing interlayer which, when incorporated into a laminated glazing, provided good optical quality.